1. Field of the Invention
The present invention relates in general to lens barrels, and in particular to helicoid threaded members forming part of a lens barrel.
2. Related Background Art
Lens barrels usually have a structure for allowing the optical system to be movable in the axial direction. In many cases, the structure comprises helicoid threaded tubes which are connected together by means of their threads of substantially trapezoidal cross section formed helically on the inner or outer surfaces thereof.
In the past, helicoid threaded tubes of this type were formed of a metallic material through high precision machining. Recently, with the development of plastics molding techniques, it has become possible to produce plastics tubes of this type having threads integrally formed therewith by molding, whereby reduction in cost and weight of the tubes has been achieved.
These helicoid threaded tubes, however, suffer from a problem in that burrs inevitably occur during molding at portions thereof corresponding to matching surfaces of the parts of the metal split mold used to form them. When plastics articles are formed with the use of a split mold, burrs will occur depending on various conditions such as the accuracy of the split mold, errors in matching of the mold halves, and abrasions in the mold. The occurrence of such burrs would virtually be unavoidable even if the above-mentioned conditions are controlled carefully. It takes a long working time, and therefore necessitates a substantial cost, to remove burrs formed in the thread.
To solve these problems, the threads of a prior art plastics helicoid threaded tube, integrally formed with the tube, are configured such that the height and width of the threads decrease gradually in the portion on the inner or outer surface of the tube where burrs usually occur so that the portions of the threads having gradually decreasing height and width can avoid engagement with the corresponding threads of another threaded tube when these tubes are connected together by means of their threads, whereby the tubes can smoothly move or slide on each other.
FIGS. 12-16 show prior helicoid threaded tubes having threads which are partly configured in a manner as described immediately above. Specifically, FIG. 12 is a cross sectional view of a male helicoid threaded tube, and FIG. 13 is a cross sectional view of a corresponding female helicoid threaded tube.
The male threaded tube 30 shown in FIG. 12 has threads in an outer peripheral portion thereof which are shown between an interrupted and the outer contour. Two grooves 3c and 3d, diametrically opposed to each other, are straight keyways which serve to prevent the rotation of the male threaded tube 30 when the female helicoid threaded tube connected thereto is rotated to move the male tube 30 forward or backward along the optical axis. As shown in FIG. 12, the height of the threads decreases gradually in portions 34 and adjacent regions, where the threads intersect with parting lines on the helicoid threaded tube to which the matching surfaces A, B, C and D of the split mold parallel to the optical axis are positioned, so that molding burrs, if any, in these portions will not engage with the threads of the female helicoid threaded tube.
FIG. 14 shows the cross section, taken along a transverse plane which is parallel to the optical axis, of the threads at the split line portions 34. The threads are substantially lower and narrower in these portions than in other portions M, shown in FIG. 12, where the mold matching surfaces are not positioned. The thread height in the portions M is indicated by an interrupted line in FIG. 14.
The female helicoid threaded tube 32 shown in FIG. 13 has threads which are shown lying between an interrupted line and its inner contour. As shown, the threads have a height which decreases gradually in split line portions 35 and adjacent regions, which portions 35 are formed by making six radially diverging matching surfaces E, F, G, H, I and J of the split mold intersect with the threads, so that molding burrs, if any, will not engage with the threads on the male helicoid threaded tube.
In the split line portions 35 the threads have a cross sectional shape which is substantially identical to that of FIG. 14, and are substantially lower and narrower in these portions than in other portions N shown in FIG. 13 where the mold matching surfaces are not positioned.
In the prior art male and female helicoid threaded tubes respectively formed as a single piece member by molding, in which the threads are reduced in both height and width in the split line portions where the mold matching surfaces are positioned, so that the top and side surfaces of such thread portions of one threaded tube can avoid engagement with the relative positions of the threads on the other threaded tube, the thread portions 34 and 35 of reduced height and width depends on the relative angular positions of the threaded tubes. The thread portions of reduced height and width are hereinafter called "relief thread portions" or simply "relief portions".
In FIG. 15, in which the male and female helicoid threaded tubes 30 and 32 are simply shown superposed, the threads of these tubes engage together in angular positions or ranges K1, K2, K3, K4, K5 and K6. In the other angular ranges, however, at least part of the threads are not in engagement with corresponding threads.
When a mechanical impact is applied to one of the helicoid threaded tubes being in such a relative position, the sides of the thread portions engaging together will be subjected to enhanced stress. Helicoid threaded tubes of plastics have a lower modulus of elasticity than those of metal, and therefore have a stronger tendency to be deformed under stress than metallic threaded tubes. For this reason, the engaging portions of the threads are then deformed and the so deformed portions will interfere with the relative movement of the connected helicoidal threaded tubes.
To avoid such problem, the thickness of the threaded tubes should be increased so that deformation of the threads can be prevented, or the angle of relief of the relief portions of the threads, which are not in engagement with corresponding thread portions, should be made smaller. Unfortunately, a larger thickness of the tubes is associated with a larger diameter of the lens system mounted on the tubes. Further, to make the angle of relief smaller, the height of the threads must be steeply decreased from their effective outer diameter 31 or from effective inner diameter 33 in the relief portions 34 and 35, making rough the outer or inner contour of the threaded tubes. This roughness also causes the action of rotating the connected tubes to be not smooth, and consequently skill or experience is necessitated to precisely position the lens system. One of the male and female helicoid threaded tubes may be made of metal and the other of plastics, the metal one being provided with threads formed by machining. In this case, the threads of the tube of plastics must still have relief portions where molding burrs tend to occur. The existence of the relief portions necessarily causes a decrease in the total area of contact between engaging threads and also decreases the mechanical strength of the threads of plastics. Thus, the threads of these tubes made of different materials may be likely to be disengaged, as in the case of tubes both of which are made of plastics.
Further, in the relative position shown in FIG. 15, the parting line segments of one helicoid threaded tube are located in angular positions different from those in which the parting line segments of the other helicoid threaded tube. At least one relief portion of one tube will be aligned with one relief portion of the other tube when the relative position of the tubes is changed by a small extent of relative rotation. Since the height of the relief portions of the threads is lower than the other portions of threads, there will be a clearance where relief portions of the threads on one tube are aligned or registered with, i.e. located at the same angular positions of, relief portions of the engaging threads on the other tube connected to the one tube. Rays of light can pass through such clearance, as shown in FIG. 16. If the helicoid threaded tubes are used in a photographic lens barrel, then there is a danger that leakage light passing through such clearance can reach the photosensitive film surface, degrading the picture taken.